1. Field of the Invention
The present invention relates to zoom lens systems, and more particularly to temperature-compensated zoom lens systems provided with the functions of correcting focus deviation accompanying variation in temperature and of correcting chromatic aberration.
2. Description of Related Art
To obtain a high-resolution optical system, it is essential to suppress chromatic aberration. In correcting the secondary spectrum of chromatic aberration, materials with high anomalous dispersion are very effective. These materials, however, exhibit large variation in refractive index accompanying variation in temperature, and thus tend to suffer from the problem of focus deviation. Patent Documents 1 to 4 listed below propose optical systems addressing the problem.
For example, according to Patent Document 1, with a view to reducing chromatic aberration and simultaneously reducing focus deviation accompanying variation in temperature during zooming, in a lens unit other than a focusing unit, at least one positive lens element made of an anomalous dispersion glass with low refractive index and low dispersion and at least one negative lens element made of a glass with high refractive index, high dispersion, and small temperature-related variation in refractive index are used to build the proposed zoom lens system. According to Patent Document 2, the thermal expansion of a lens barrel is exploited to correct focus deviation in the proposed zoom lens system. In this zoom lens system, a lens barrel that couples a plastic lens element and an image sensor together is made of a material with a linear expansion coefficient corresponding to the displacement of the focus position. Thus, as temperature rises, the lens barrel expands along the optical axis, and thereby cancels the focus deviation resulting from variation in the refractive index of the lens element.
Patent Document 3 discloses a zoom lens system in which conditions related to focal length and anomalous dispersion are so set as to reduce chromatic aberration and focus deviation. Patent Document 4 discloses an optical system in which the focus deviation occurring in anomalous dispersion glass provided for correction of chromatic aberration is corrected for with quartz glass.
Patent Document 1: JP-A-H8-297244
Patent Document 2: JP-A-2003-248171
Patent Document 3: JP-A-2008-242402
Patent Document 4: JP-A-2011-53663
Inconveniently, however, none of the optical systems disclosed in Patent Documents 1 to 4 can correct chromatic aberration and focus deviation both satisfactorily. For example, the zoom lens system disclosed in Patent Document 1 provides only a small effect of correcting both chromatic aberration and focus deviation. The zoom lens system disclosed in Patent Document 3, employing no or only one anomalous dispersion glass lens element, can suppress focus deviation but is hardly effective in correcting chromatic aberration.
The zoom lens system disclosed in Patent Document 2 may encounter cases such as where, for the sake of compactness, the lens barrel cannot be made long enough along the optical axis to secure the desired expansion stroke and where, for the sake of structural strength, the lens barrel cannot be made of a material with a proper linear expansion coefficient. This limits the extent to which focus deviation can be corrected, and thus makes it difficult to correct focus deviation satisfactorily. Another problem is that it is not possible to reduce variation in zooming-induced focus deviation between different focal lengths. The optical system disclosed in Patent Document 4 is directed to a single-focal-length lens system. Thus, Patent Document 4 teaches nothing about a method of correcting variation in focus deviation between different focal lengths which is peculiar to zoom lens systems.